Currently only used when packet split mode is enabled with jumbo frames,
IP payload checksum (for fragmented UDP packets) is mutually exclusive with
receive hashing offload since the hardware uses the same space in the
receive descriptor for the hardware-provided packet checksum and the RSS
hash, respectively.  Users currently must disable jumbos when receive
hashing offload is enabled, or vice versa, because of this incompatibility.
Since testing has shown that IP payload checksum does not provide any real
benefit, just remove it so that there is no longer a choice between jumbos
or receive hashing offload but not both as done in other Intel GbE drivers
(e.g. e1000, igb).

Also, add a missing check for IP checksum error reported by the hardware;
let the stack verify the checksum when this happens.

CC: stable <stable@vger.kernel.org> [3.4]
Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>